A continuing trend in the electronics industry is the miniaturization of electronic circuits and a corresponding increase of the circuit element density of electronic circuits. On conventional printed circuit boards, a large fraction of the board surface area is occupied by surface-mounted passive electrical devices, such as resistors, capacitors and inductors. One way to increase the density of circuit elements in an electronic circuit is to remove passive devices from the surface of the circuit board, and embed or integrate the passive devices into the circuit board itself. This has the added advantage of placing the passive devices much closer to the active circuit components, thus reducing electrical lead length and lead inductance, improving circuit speed, and reducing signal noise. Signal noise and power bus noise can lead to signal integrity and electro-magnetic interference (EMI) issues. Embedding the passive components into the board can reduce the size, thickness and number of layers required in the board, which can significantly reduce the cost of the circuit board. The reduction in board size and thickness, as well as the elimination of the surface-mounted components and their associated vias and solder joints, can provide a significant reduction in weight and improved reliability.
Complex high speed digital and mixed signal systems are being used to accommodate the desire for increasing functionality in electronic products. These high speed digital systems require large quantities of power supply decoupling capacitors to effectively decouple the AC noise from the power supply. Additionally, signal rise times, frequencies and current and board densities continue to increase, as operating voltages and their associated noise margins decrease. Simultaneous switching noise (SSN) is increasing due to lower operating voltages and higher currents. These factors make power supply decoupling more challenging.
Even with the use of embedded passives, average printed circuit board layer counts are increasing, which is increasing overall board thickness. Increased board thicknesses and higher assembly temperatures, which are needed to accommodate regulatory-required lead-free assemblies, significantly increase thermal stresses and moisture vapor pressures on the board materials and components. Complex board designs must endure more assembly passes due to the increase in double sided surface mount components and increased rework due to higher component densities. To survive the increased number of assembly passes and harsh conditions, board materials must have good adhesion, high decomposition temperatures and high resistance to moisture sensitivity.
Additionally, recent regulatory requirements have specified the use of materials with less than 900 ppm of bromine and less than 900 ppm of chlorine in printed circuit boards, modules and chip packaging. Thus, embedded passive materials must not only have excellent electrical, mechanical, thermal properties and high reliability but are also limited by composition to not include more than trace amounts of halogens.